This invention relates to the art of air filtering apparatus, and more particularly to air filtering apparatus in which a reverse air stream blowing in a direction contrary to the direction of flow of the filtered air stream is intermittently directed through the filter medium of the apparatus to dislodge any foreign matter accumulating on the filter medium surface.
A variety of air filtering apparatus has long been known and used in connection with industrial facilities where the industrial processes involved produce undesirable quantities of airborne foreign material. Thus, such air filtering apparatus finds relatively widespread use in lumber mills, furniture factories, textile mills and the like.
Among the air filtering apparatus which has been evolved for use in this connection are so-called "reverse cleaning bag filters" in which the air to be filtered is collected in a collecting air chamber and passed through the walls of a dust bag, with the walls of the dust bag providing a filter medium entraining the foreign matter in the air stream. Thereafter, the filtered air moves from the dust bag to a filtered air chamber which is maintained at an air pressure lower than that of the collecting chamber, so as to insure the flow of air from the collecting air chamber through the dust bags into the filtered air chamber. A plenum is mounted in the filtered air chamber to intermittently direct air in a reverse flow direction (that is in a direction counter to the flow of the filtered air stream) through the dust bag to displace any filtered material accumulating on the surface of the dust bag. Swanson, in U.S. Pat. Nos. 2,844,216 and 2,974,748, discloses filtering apparatus of the type here involved. As disclosed in these Swanson patents, the filtering apparatus is formed with a lower collecting air chamber in which the air containing the foreign matter is received. A filtered air chamber is arranged above the collecting air chamber, and these chambers are separated by a grid plate containing a plurality of air passages. Dust bags are arranged beneath the grid plate, with the dust bags extending into the collecting air chamber. The air to be filtered is fed into the collecting air chamber, passed through the side walls of the dust bags in the collecting air chamber, then up through the interior of the dust bag through the air passages in the grid, and into the filtered air chamber from which it is discharged. In the filtered air chamber, means are provided for intermittently directing a reverse flow of air from the filtered air chamber down through the filter bags, back into the collecting air chamber. This reverse air flow serves to dislodge any filtered material accumulating on the surface of the dust bags and interfering with desired filtering action.
In the Swanson U.S. Pat. No. 2,844,216, the means for providing for reverse air flow comprises a cylinder and a piston mounted therein for reciprocatory movement, with the cylinder outlet subject to movement into alignment with one of the dust bag outlets so that the piston may drive air from the cylinder into the dust bag in a reverse flow direction.
In the later Swanson U.S. Pat. No. 2,974,748, reverse flow is provided by a manifold moveably coupled to a discharge plenum coupled to a blower mounted on top of the filtered air chamber, which is additional to the blower employed to effect air movement into the collecting air chamber. The manifold is mounted for movement over the grid plate and leads to a casing containing a valve, which is intermittently opened to permit air to flow from the manifold in a reverse direction through the filter bag. This later Swanson development serves to eliminate the problems in maintaining and operating the piston and cylinder arrangement of the earlier Swanson apparatus. However, as will be apparent to those skilled in the art, the Swanson arrangement shown in the later U.S. Pat. No. 2,974,748 presents a number of manufacturing and maintenance problems, serving to significantly increase the cost of manufacture and maintenance.
Thus, Swanson provides a relatively complex air path in which the direction of air flow is changed several times as it flows from the reverse air blower to the air chamber to the return air manifold creating inefficiency of air transfer between the blower and filter, increasing production, operation and maintenance costs. Additionally, Swanson employs a relatively complex valve structure for controlling the flow of air from the manifold to the dust filter, increasing costs of production and maintenance and providing areas of breakdown.
Additional problems arise in the use of the previously evolved air filtering apparatus of the type above discussed in delivering desired quantities of reverse flow air to the dust bags. To this end, the moving reverse air source, such as the manifold and valve, or piston-cylinder arrangements of Swanson must move over the grid plate between the collecting chamber and filtered air chamber effecting a relatively air tight flow path between the reverse air source and the filter bag outlet. Any foreign material inadvertently accumulated on the grid plate tends to interfere with the free movement of the rigid valve housing or cylinder of Swanson over the grid plate, so that a loosened screw or washer, or an accumulation of foreign matter not retained by the filter bags can often produce jamming of the valve housing or cylinder, interfering with the desired air tight reverse air flow path.
Additionally, previously evolved filtering apparatus has generally required that the dust or filter bags be removed from the lower collection chamber. Since these collection chambers are relatively dirty as compared to the filtered air chamber, and in practice are often 6 to 10 feet in height, it is necessary for a maintenance worker to reach up in a relatively dirty environment to replace the filter bags, creating an uncomfortable and inefficient working situation.
Further, previous reverse cleaning filters have employed grid plates formed of a unitary sheet, so that damage to any part of the grid plate requires replacement of the entire plate.